A method of formation of a color image which is generally used in the field of a silver halide color photographic material is a method of forming an azomethine dye by reacting an oxidized aromatic primary amine based color developing agent with a coupler using an exposed silver halide as an oxidizing agent. A subtractive color process is used for reproducing a color image in such a method, and a color image is in general formed by changing the amounts of dyes formed of three colors of yellow, magenta and cyan.
1H-pyrazolo[1,5-b][1,2,4]triazole based magenta couplers are particularly superior as magenta couplers above all in not only an absorption characteristic of a dye but also a coloring ability and fastness.
The absorption characteristic of a color dye is expressed in detail by a spectral absorption waveform, and can be expressed by characteristic values such as a maximum absorption wavelength and an extinction coefficient. The presence or absence of a side absorption, the width of an absorption band and the sharpness of an absorption edge, as well as the above characteristic values, are important factors for the color reproduction of color photographs. White light comprises blue light (400 to 500 nm), green light (500 to 600 nm) and red light (600 to 800 nm), and reddish purple color which is a complementary color of green light appears when green light is absorbed by a magenta dye, but when there is an absorption characteristic failure of a magenta dye, a part of blue light and red light is absorbed, leading to turbidity of a hue. Such a dye cannot reproduce clear colors. A pyrazolotriazole type coupler does not show a side absorption in a blue light region, an absorption edge of a long wavelength side is sharp, and an extra absorption in a red light region is extremely small, compared with a pyrazolone type coupler.
However, a dye having such an absorption characteristic causes a new problem when an absorption wavelength is not appropriate. That is, too short an absorption wavelength cannot reproduce clear purple and black colors because there is generated a region between the absorption region of a magenta dye and that of a cyan dye which cannot sufficiently absorb light. On the contrary, too long an absorption wavelength cannot reproduce clear red and black colors because there is generated a region between the absorption region of a magenta dye and that of a yellow dye which cannot sufficiently absorb light. Accordingly, a technique of controlling a hue has a more important meaning in the case of a pyrazolotriazole type coupler than in the case of a pyrazolone type coupler.
The absorption wavelength of the dye formed differs according to the kind of substituent used, even in couplers having the same skeleton. Therefore, a hue can be controlled by a substituent. In general, the introduction of an electron attractive group causes the shift of a wavelength to a long wavelength and the introduction of an electron donative group causes the shift of a wavelength to a short wavelength, although the degree differs according to the position to which the substituent is introduced. However, such changes of substituents are sometimes accompanied by changes of other important performances of couplers such as a coupling activity and fastness to heat and light, therefore, the selection of the couplers is often limitative.
These performances do not necessarily change simultaneously with preferable performances. In practice, couplers that are excellent on an average in various performances must be selectively used under present conditions.
The absorption wavelength of a dye can be controlled to a certain degree according to the way of use, for example, the kind and amount of a high boiling point organic solvent for use in dispersion, and the use of other additives, even when the same couplers are used. The most important thing in controlling a hue by a high boiling point organic solvent is the polarity of a high boiling point organic solvent, such as a dielectric constant, hydrogen bonding and electron donative properties. A dielectric constant is often used as the parameter of the polarity among all. A high boiling point organic solvent having a high dielectric constant causes the shift of a dye absorption wavelength to a long wavelength and a high boiling point organic solvent having a low dielectric constant causes the shift of a dye absorption wavelength to a short wavelength. The molecular structure that makes the dielectric constant high is a structure which contains many aromatic rings or polar groups containing hetero atoms. Examples of such high boiling point organic solvents that are frequently used include tricresyl phosphate and dibutyl phthalate. On the other hand, the molecular structure that makes the dielectric constant low is a structure which contains many hydrophobic aliphatic chain moieties, for example, trioctyl phosphate and fatty acid alkyl esters. Controlling of a hue by means of such high boiling point organic solvents has been succeeded to some degree, although the coloring performance of a coupler or the stability of a dye are sometimes influenced.
In addition, it is known that the addition of the compound having a phenolic hydroxyl group or a sulfonamido group can shift the absorption wavelength of a dye to a long wavelength. However, an additive that can shift the absorption wavelength of a dye to a short wavelength has not yet been known.
The 1H-pyrazolo[1,5-b][1,2,4]triazole coupler having a tertiary substituent at the 6-position and an aryl group at the 2-position, which is disclosed in EP 571,959, is stable against the composition variation of a processing solution in development. The absorption wavelength of the dye formed from this coupler by color development has an inclination to be too long for a magenta color, and it is useful to disperse the coupler in a high boiling point organic solvent having a low dielectric constant to shift the hue of the dye formed to an appropriate wavelength range.
However, it has been revealed that the use of a high boiling point organic solvent having a low dielectric constant with the above 1H-pyrazolo[1,5-b][1,2,4]triazole coupler causes a new problem of lowering of a yellow color density. The lowering of a yellow color density caused therein cannot be improved so much by increasing a coating amount of a yellow coupler or a silver halide, and this has been a large impediment in designing a silver halide color photographic material.